The present invention relates generally to microwave devices and more particularly to non-reciprocal microwave devices, such as circulators and isolators, having a large bandwidth.
Circulators and isolators are special classes of microwave devices which, like other transmission line devices, function to transfer or guide energy from one point to another. Such devices are normally classified on the basis of the field configurations called "modes" which they transmit. Devices are generally divided into groups, those capable of transmitting transverse electromagnetic (TEM) modes and those able to transmit higher-order modes. In a TEM mode, the electric field (E) and the magnetic field (H) are entirely transverse to the direction of propagation with no component of either E or H in the direction of transmission. The higher-order modes have components of E or H in the direction of transmission. The circulators and isolators of the present invention are particularly for TEM mode transmission.
Circulators and isolators are well-known microwave devices for transferring energy between two or more ports. At least one input port receives electromagnetic energy and efficiently transfers that energy to an output port. Electromagnetic energy, if any, appearing as an input at the output port is not efficiently transferred, however, to the input port. Hence, the connection of electromagnetic energy among the various input and output ports of circulators and isolators is said to be non-reciprocal and the devices function to isolate input ports from output ports.
In a circulator, the direction of the magnetic field controls the direction of circulation of electromagnetic energy. For example, in a 3-port circulator, when the magnetic field is in a first direction normal to the plane of propagation, circulation among the ports numbered one, two and three is clockwise from one to two, from two to three, and from three to one and when the magnetic field is in the opposite direction, the circulation is from one to three, from three to two and from two to one.
When one of the ports of a 3-port circulator is terminated with the characteristic impedance of the device, the device functions as an isolator. With such a termination, the performance between the remaining two non-terminated ports is the same as for a 3-port circulator.
The performance of isolators and circulators is measured in terms of the insertion loss which is a measure of the efficiency of transmission from the input port to the output port. Another measure of performance is the return loss (or VSWR) which is a measure of how well the circulator is matched to the characteristic impedance. Also, the performance is measured in terms of the isolation of the input port from the output port. Another measure of the performance is the operating bandwidth over which the device may be employed for effective transfer of energy.
Good performance is achieved when the device has a low insertion loss, high return loss (low VSWR) and high isolation over a broad band of frequencies.
Since circulators and isolators and other microwave devices frequently find applications in airborne and satellite vehicles, small size and light weight are important objectives for these devices.
The basic structure and the theory of operation of circulators and isolators has been described in the literature for many years. For example, one well-known article is "Operation of the Ferrite Junction Circulator" by Fay and Comstock, IEEE Transaction on Microwave Theory and Technique, January, 1965, pages 15 through 27. In that article, a basic 3-port strip-line circulator is described. Such 3-port circulators are sometimes called Y-junction strip-line circulators. Other multi-port circulators are also described.
It is well known from that article and otherwise that strip-line circulators basically include a center conductor located equidistant between a first ground plane on one side and a second parallel ground plane on the other side. A layer of material, such as ferrite, exhibiting gyromagnetic properties, is located between the center conductor and one ground plane and similarly a second layer of the same ferrite material is located between the center conductor and the other ground plane. In a 3-port circulator, the center conductor is normally configured in the shape of a Y with three legs, each positioned in a flat plane projecting outwardly from a center point at angles of 120 degrees. The ferrite layers lie in a plane parallel to the plane formed by the center conductor and the two ground planes. A magnetic field is applied perpendicular to these planes through the ferrite layers and the center conductor. The magnetic field functions to bias the ferrites near but not at their saturization magnetization. These devices are known as below resonance devices.
In circulators of this well known type, the bandwidth tends to be limited due to high insertion losses at the low frequency end of the pass band and tends to be limited due to higher-order moding (non TEM transmission) at the high frequency end of the pass band. Operation of such circulators is in a single perfect circulation mode.
The bandwidth is defined to be the difference between the highest frequency of operation and the lowest frequency of operation at which acceptable levels of insertion loss without higher-order moding can be achieved. The bandwidth is given as follows: EQU BW=f.sub.h -f.sub.l Eq.( 1)
where:
BW=bandwidth PA1 f.sub.h =highest frequency of pass band PA1 f.sub.l =lowest frequency of pass band PA1 %BW=percent bandwidth relative to center frequency PA1 f.sub.h =high frequency of pass band PA1 f.sub.l =low frequency of pass band PA1 f.sub.c =center frequency of pass band=(f.sub.l +f.sub.h)/2
In microwave devices, bandwidth is frequently measured in terms of the percent of the center frequency of the pass band as follows: EQU %BW=(100)(f.sub.h -f.sub.l)/f.sub.c Eq.( 2)
where:
If the bandwidth is one octave, that is f.sub.h =2 f.sub.1, then the percent bandwidth is as follows: EQU %BW=100(f.sub.1)/(3f.sub.l /2)=200/3=66.66% Eq.(3)
From Eq.(3), it is clear that a one octave bandwidth is equal to 66.66 percent bandwith. For example, a microwave device which operates over a bandwidth of 2 through 4 GHz (10.sup.9 Hz) operates over one octave since the higher frequency is twice the lower frequency. The percent bandwidth is determined by Eq.(3) where the center frequency is 3 and the bandwidth is 2 so that the percent bandwidth is (2/3)(100) or 66.66%. For a device in which the bandwidth extends over two octaves, for example from 2 through 8 GHz, the center frequency is 5, and the bandwidth is 6 and the percent bandwidth is (6/5)(100) or 120 percent.
Strip-line circulators have been available with acceptable insertion loss and isolation over one octave (66 percent bandwidth). There is a need for it and it is an object of the present invention to provide improved devices having low insertion loss and high isolation up to 100 percent bandwith or more and particularly to provide such devices which are compact and light weight.
It is still another object of this invention to provide a 100 percent or more bandwidth circulator and/or isolator that may be easily manufactured.
It is another object of this invention to provide a 100 percent or more bandwidth circulator and/or isolator for microwave transmission which will handle relatively high power of fifty watts or more.
It is an additional object of this invention to provide a device which functions equally well as a circulator or as an isolator.